1. Field of the Invention (Technical Field)
The present invention relates to high-sensitivity detection of contaminants in gases by optical techniques generally termed photoacoustic spectroscopy (PAS) or optoacoustic spectroscopy.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Photoacoustic spectroscopy is a known technique for high sensitivity detection of trace gases. Absorption by the target species of incident optical energy results in a transient heating of the gas. If the incident optical energy is modulated then the gas is periodically heated which creates a time-varying pressure wave or sound. The sound can be measured with a microphone.
PAS is often enhanced through the use of acoustically resonant gas cells. These cells build up sound intensity at the resonance frequencies. Depending on the type of noise dominant in the system, resonant cells can dramatically improve signal-to-noise ratios and thereby, the measurement sensitivity. Individual and combination acoustic resonance modes including longitudinal, radial, and azimuthal are often utilized. Unfortunately, resonance frequencies depend on the local speed of sound which can change with temperature and gas composition. In addition, changes in cell dimensions due to mechanical stress can change these resonance frequencies. Thus, for a practical resonance-based photoacoustic spectrometer, it is necessary to maintain the modulation frequency at the acoustic resonance frequency.
The ability to maintain the optical source modulation frequency or its harmonics on an acoustic resonance frequency of a photoacoustic cell will hereafter be referred to as resonance frequency locking. In field measurements, microphone noise is not usually a photoacoustic instrument""s sensitivity-limiting noise source. When microphone noise is not the limiting noise source, operation on an acoustic resonance enhances the signal-to-noise ratio. Optoacoustic Spectroscopy and Detection, Y-H Pao, ed. (Academic Press, New York, 1977), pps. 20-22. Because there is a 1/f dependence of the photoacoustic signal on frequency, operation at a resonance frequency represents a compromise between the enhancement, or Q, of the resonance cell and higher frequency operation. Thus, resonant operation is usually desirable provided the resonance frequency is not so high that the 1/f penalty outweighs the cavity Q. For example, a cell with a resonance at 8000 Hz with a Q of 200 would generate the same signal, all other parameters being equal, as a non-resonant cell operating at 40 Hz. In many systems, there is less background acoustic noise at 8000 Hz than at 40 Hz; resonant operation at 8000 Hz then provides a better signal-to-noise ratio.
Acoustic resonances may have narrow bandwidths. As the amplification factor (or resonance cavity Q) increases for a given frequency, the bandwidth gets narrower. Thus, as sensitivity is increased by improving the acoustic quality of the photoacoustic cell, the need for a method to maintain the acoustic modulation frequency on resonance increases proportionately. For example, a cell with a Q of 200 at 8000 Hz will have a bandwidth of 40 Hz. Thus, a change of the resonance frequency of only 20 Hz will reduce the signal by a factor of 2. A change of 20 Hz can be caused by a temperature change of less than 1 degree C at room temperature.
The present invention achieves continuous, real-time acoustic resonance frequency locking by sweeping the optical source modulation frequency or its harmonics across a cell acoustic resonance. Because the frequency of the modulation is swept over the cell resonance, the amplitude of the acoustic signal is modulated. The acoustic resonance converts the frequency modulation into an amplitude modulation. This effect is similar to frequency or wavelength modulation where a laser is swept across a molecular absorption feature where there is a maximum attenuation of the beam on the peak of the molecular absorption. With the present invention, there is a maximum amplification or enhancement on the peak of the acoustic resonance feature.
With analogy to wavelength modulation, if the acoustic modulation frequency is swept equally to either side of the peak of the acoustic resonance, a sweep frequency of xcfx89 will result in an amplitude modulation at a frequency of 2xcfx89 since the sweep will cross the resonance center twice during every sweep cycle. When the frequency sweep is symmetric about the acoustic resonance line center, the carrier, the 2xcfx89 and higher even harmonic signals will be at maximum and the 1xcfx89 and higher odd harmonic signals will have zero crossings. Thus, an odd harmonic of the frequency modulation sweep rate can be used as an error signal for adjusting the frequency modulation carrier frequency. As the acoustic resonance frequency drifts with temperature, gas composition, etc., the acoustic modulation frequency will be continually updated and maintained to match the cell acoustic resonance frequency.
The present invention for acoustic resonance frequency locking can be used to equal effectiveness regardless of the method of producing photoacoustic signals. In traditional PAS the optical source radiation is amplitude modulated (AM). The modulation can be achieved by means of a mechanical chopper, a shutter, an acousto-optic modulator, or modulation of a (e.g., semiconductor) pump waveform. Other methods for achieving an amplitude modulated optical source are contemplated by and fall within the invention. In addition, Southwest Sciences, Inc. has implemented wavelength modulation spectroscopy (WMS) with PAS detection, as described in U.S. patent application Ser. No. 09/687,408. With WMS, the optical radiation source is modulated in wavelength, not amplitude (if WMS is implemented with injection current modulation of a diode laser, AM results only as a side-effect). Nevertheless, WMS produces a synchronous amplitude modulated pressure wave at the microphone. Because the present acoustic resonance frequency locking mechanism depends on features of the cell acoustic resonance and not the source of the sound, it is equally applicable to AM and WMS-based PAS.
A source of acoustic power independent of PAS generation (a speaker) has previously been used to implement an acoustic frequency locking mechanism. M. W. Sigrist and coworkers generated sound at a resonance frequency of a PAS cell with a speaker whose frequency was locked to a cell resonance via the microphone""s detection phase at the resonance frequency. G. Z. Angeli, et al., xe2x80x9cDesign and characterization of a windowless resonant photoacoustic chamber equipped with resonance locking circuitryxe2x80x9d Rev. Sci. Instrum. 62, 810 (1991). The locked resonance frequency was used to generate an amplitude modulated optical frequency for PAS generation The optical source modulation operated at a separate cell resonance that was a constant fraction of the frequency used for resonance locking. Several disadvantages of this approach are readily apparent. The method requires a separate acoustic source independent of the PAS generation source. The method introduces sound at frequencies other than that where the PAS signal occurs. This sound must be attenuated in order to prevent overloading of the detection microphone. Depending on the acoustic source spectral purity, noise may be induced at the PAS detection frequency. The method relies on the PAS resonance frequency and the acoustic source generated locking frequency changing in the same way in a dynamic environment. If the cell geometry changes differently for the two frequencies (due, for example, to a mechanical stress), the frequency ratio will not be constant and the lock will be lost.
M. W. Sigrist and coworkers also implemented a type of acoustic resonance maintenance by scanning over the cell resonance. A. Thony, et al., xe2x80x9cNew Developments in CO2-Laser Photoacoustic Monitoring of Trace Gases xe2x80x9d, Infrared Phys. Technol. 36, 585 (1995). Their method consisted of a slow scan of several discrete steps over the cell resonance. The PAS signal vs. acoustic frequency was fit to an inverted parabolic curve and the peak of the fit used as the resonance line center. The acoustic modulation frequency was then corrected to coincide with the calculated resonance line center. This method was then repeated approximately every 20 minutes. Obviously, this is not a real-time resonance frequency lock. If the cell resonance drifted during the 20 minute interval between measurements, the PAS source modulation frequency would not correspond to the cell resonance frequency. In addition, sample signal acquisition is halted during the resonance scan measurement and fitting. Continuous sample measurement is precluded in this approach. The method has utility is removing very slow drift effects, but does not provide high-fidelity resonance locking for a dynamically changing PAS sample environment. Contrast that with the present invention, whereby continual modulation over the cell resonance provides a real-time error signal for perpetual operation on the cell resonance with continuous PAS sample analysis.
The invention is of a photoacoustic spectroscopy method and apparatus for maintaining an acoustic source frequency on a sample cell resonance frequency comprising: providing an acoustic source to the sample cell, the acoustic source having a source frequency; detecting the acoustic power within the cell; repeatedly and continuously sweeping the source frequency across the resonance frequency at a sweep rate; and employing an odd-harmonic of the source frequency sweep rate to maintain the source frequency sweep centered on the resonance frequency. In the preferred embodiment, sound is generated from absorption of optical power by a species inside the sample cell, preferably frequency or wavelength modulated optical power. The invention can be employed with a flowing gas absorber species. Alternatively, sound may be generated from a speaker. A metric proportional to acoustic power inside the cell (and also to concentration of the absorber gas species) is measured, such as by measuring acoustic power occurring at an even-harmonic of the sweep rate or by measuring amplitude of a frequency modulation carrier. The odd harmonic determination is preferably done via a lock-in circuit, which may incorporate digital signal processing technology. The waveform of the acoustic source frequency is preferably sine, triangle, square, or quasi-square.
The invention is also of an acoustic resonance frequency locked photoacoustic spectrometer comprising: an acoustic source repeatedly and continuously sweeping an acoustic source frequency across a resonance frequency of a sample cell at a sweep rate; and a lock-in amplifier employing an odd-harmonic of the acoustic source frequency sweep rate to maintain the acoustic source frequency (or one of its sub-harmonic frequencies) sweep centered on the resonance frequency.
A primary object of the present invention is to provide for continuous, real-time, acoustic resonance frequency locking in PAS.
A primary advantage of the present invention is that it can operate at high frequencies and can be used in measurement of flowing gaseous species.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.